This invention relates to a process for converting solid carbonaceous materials such as coal into liquid hydrocarbonaceous products. More particularly, this invention relates to an improved process to effect hydrogenation of coal to provide hydrocarbonaceous liquids suitable for use as substitutes for petroleum liquids.
Liquefaction of coal and similar solid carbonaceous materials is known as a method for producing hydrocarbonaceous liquids. A liquid product from liquefaction conversion operations may be substituted for petroleum fuels and be refined to produce gasoline, etc., in a manner analogous to that used to refine petroleum. Because of the abundance of coal reserves and decreasing petroleum resources, it is becoming increasingly important to develop practical methods for deriving petroleum substitutes from coal.
In a typical operation for solvent liquefaction of coal, the coal is pulverized and mixed with a hydrocarbonaceous solvent. The mixture of coal and solvent, generally with added hydrogen gas, is subjected to heat and pressure. The solid to liquid conversion reaction involves the hydrogenation and hydrocracking of the various unsaturated hydrocarbon molecules of coal to effect an increase in the ratio of hydrogen to carbon. As the ratio of hydrogen to carbon increases for any given molecule the hydrocarbon molecule becomes more liquid and less viscous. Typically, coal exhibits an average hydrogen to carbon ratio of 0.8, that is 0.8 hydrogen atom per carbon atom. In the liquefaction process there is an attempt to increase this hydrogen to carbon ratio to about 1.0 or 1.2.
Since the coal to be liquified is relatively low in hydrogen content it is necessary to add a significant amount of hydrogen to the coal during the liquefaction operation. The prior art has devised several methods for facilitating the necessary hydrogen addition involved in the conversion reaction. Of these, the most commonly suggested method has involved the use of extrinsic catalysts in combination with a hydrogen-donor solvent.
The use of catalyst additives has been found to be generally very expensive. These catalysts rapidly become poisoned by the metals present in the coal, and are quite difficult to separate from the solid residue remaining after the extraction operation.
Hydrogen-donor solvents have also provided a partial solution to the problem of transferring hydrogen into the coal. These solvents such as creosote oil, petroleum middle oil, tetralin and coal derived process solvent, serve to donate hydrogen to the coal molecules and simultaneously revert from a partially saturated to a more aromatic structure. However, the amount of hydrogen that hydrogen-donor solvents may typically transfer into the coal is quite limited, and further hydrogenation treatment subsequent to the liquefaction operation is sometimes necessary to regenerate the hydrogen-donor solvent. It addition, some hydrogen-donor solvents which have been used in the past in a technically feasible manner have been quite economically infeasible. For example, tetralin has been used in the past as a hydrogen-donor source for the conversion of coal solids to liquid materials. Although tetralin has proved to be a good hydrogen-donor source the cost is prohibitive, considering that tetralin is in itself a highly refined petroleum product. Recycle of coal slurry derived from the process has offered a significant advance in reducing the costs associated with coal liquefaction and conserving the intrinsic catalytic activity of coal ash.
Although some of the prior art liquefaction procedures have been successful under certain conditions, the need remains for other practical and inexpensive solutions to the long felt problems concerned with coal liquefaction. Consequently, the need especially exists for a hydrogen-donor solvent with intrinsic catalytic properties to supplement the catalytic effect of coal ash.